This study is to determine the physiological and biochemical mechanisms of tachyphlaxis to infused Flolan in patients with Primary Pulmonalry Hypertension by examining the relationship between the rate of infusion of Flolan and a measurable plasma metabolite, 6-keto-PGF1a, during the initial dosing as well as during chronic administration. This study will also investigate the changes associated with the "cessation Syndrome" that occurs when infusion is abruptly interrupted and evaluate the effect acute and chronic Flolan infusion on in vivo platelet activation. We are currently calculating data from some of our normal volunteers and continue to enroll PPH patients.